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Chuang Zhao

Footprint fossils suggest lizards have been running on two feet for 110m years

Fossil bones and skeletons tend to get most of the attention in palaeontology, but sometimes we can learn more about extinct life from the footprints long-extinct animals left behind. That’s the case in new research that claims to have found 110m-year-old trackways of lizards running on two feet (bipedally). Yet interpreting what fossilised footprints actually show is never that easy.

Modern lizards are a highly successful group of reptiles, with more than 5,800 species living today. But because they are generally so small and light, lizards have left a pretty sparse fossil record. Bones and skeletons are unusual, and footprints even rarer. So the fossil trackways described in the new study, published in Nature Scientific Reports, are particularly interesting.

The study focuses on a single slab of rock from Hadong County, South Korea. On one side of the rock are two trackways next to each other, oriented in opposite directions. There are also two other trackways partially preserved at the edges of the slab. What makes these trackways so fascinating is that they consist almost entirely of impressions left by the hind-feet. There are 25 impressions and only four were made by fore feet.

Ancient footprints. Nature Scientific Reports, CC BY

The feet that made the tracks appear to have had five toes (the feet were “pentadactyl”), and match the general shape of lizard feet. The lack of fore-foot impressions led the researchers to conclude that the animals must have been running bipedally.

Today there are many lizards that can run bipedally (over 50 in fact), the most famous of which is Basiliscus basiliscus or common basilisk, also known as the “Jesus Lizard” for its ability to run over water. They tend to only run on two legs at high speed, preferring four legs for most movement. Given that living lizards can move bipedally, it is perhaps not surprising that lizards living 110m years ago could do the same.

Still, if lizards were able to run bipedally 110m years ago, it tells us that the lizards doing so today aren’t unique or particularly special for being able to do so. For dinosaurs, birds, and humans, we can see that the forelimbs are freed up for other tasks such as grasping or flying.

On the other hand, for lizards that normally walk on all fours and don’t use their hands during bipedal running, it’s not clear why some might run on two legs. One recent suggestion is that it’s an accident of evolution, with the front of the body naturally flipping up as the lizard moves faster. Researchers are still trying to work this out, and knowing that this phenomenon appears in ancient lizards adds another angle to consider bipedalism from.

Reading the record

But it can sometimes be difficult to read the fossil record directly. There are many fossils where only either the fore-feet or hind-feet have left footprints. Notably, some of these odd trackways have been left behind by the massive sauropod dinosaurs. It’s highly unlikely that a 30- or 40-ton animal was walking for significant distances on only it’s hands, and so other explanations have been put forward.

Older explanations suggest the trackways may have been left by an animal moving in deep water, whose front or hind legs were floating above the sediment surface while the other pair “punted” off the bottom. Such hypotheses have since been shown to be unlikely, and recent experiments using computer simulations have used a phenomenon known as the “Goldilocks effect” to explain such trackways.

Footprint maker. Nature Scientific Reports, CC BY

In the context of fossil tracks, this means that the weight of an animal had to be just right to leave a print, depending on what the sediment was like. Too heavy and the animal would sink. Too light and it won’t be enough to leave a mark. This effect applies even for relatively larger feet that distribute the weight more widely and so reduce the pressure, or for sediments that deform under a range of different loads.

Things get really interesting with animals that have have fore-feet and hind-feet of very different sizes, as is the case (among others) for some sauropod dinosaurs and lizards. In these cases, the fore- and hind-feet may exert very different pressures depending on where the animal’s centre of mass is located, and how large the feet are. Those pressures may fall either side of the load-bearing capacity of the sediment, and the fore-feet might sink and leave tracks while the hind-feet don’t (or vice versa).

Footprints are not feet and they can be more difficult to interpret than first appears. So, are the new lizard tracks the earliest evidence of bipedalism in lizards? Or are they a preservational artefact left by a quadrupedal animal?

Further study of the specimen might find alternative explanations for the mostly-missing fore-foot footprints. But for now, the authors of the new study make a strong case for a bipedally running trackmaker. The trackways are wide, and not unlike tracks that would be expected from a living lizard that can run on two legs. That makes these new tracks extremely interesting as a glimpse into lizard locomotion 110m years ago.

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